1. Field of the Invention
The present invention relates to non-reciprocal circuit elements and more particularly to a non-reciprocal circuit element, such as an isolator or circulator, preferably for use in microwave bands.
2. Description of the Related Art
Hitherto, non-reciprocal circuit elements, such as isolators and circulators, have characteristics of transmitting signals only in a predetermined specific direction but not in the opposite direction. By making use of these characteristics, for example, isolators are used in transmission circuit sections of mobile communication devices, such as mobile phones.
As a non-reciprocal circuit element of this type, there is known a two-port isolator with low insertion loss as described in Japanese Unexamined Patent Application Publication No. 2007-208943. As illustrated in FIG. 14, in this isolator 100, a first central electrode 135 and a second central electrode 136 (inductors L11 and L12) are arranged on the surface of a ferrite 132 so as to cross each other in an insulated state. DC magnetic fields are applied by permanent magnets (not illustrated) to portions where the first and second central electrodes 135 and 136 cross each other, so that the first and second central electrodes 135 and 136 are magnetically coupled to each other. One end of the first central electrode 135 serves as an input port P1, whereas the other end thereof serves as an output port P2. One end of the second central electrode 136 serves as the output port P2, whereas the other end thereof serves as a ground port P3. A terminating resistor R11 and a capacitor C11, which are connected in parallel with each other, are connected between the input port P1 and the output port P2. Also, a capacitor C12 is connected in parallel with the second central electrode 136. The first central electrode 135 and the capacitor C11 form a resonant circuit, whereas the second central electrode 136 and the capacitor C12 form a resonant circuit. Further, impedance adjusting capacitors CS11 and CS12 are respectively connected to the input port P1 and the output port P2. The isolator 100 also includes external connection terminals IN, OUT, and GND.
The isolator 100 is built into a transmission circuit of mobile phones. Specifically, the input-side external connection terminal IN is connected to a transmission-side power amplifier PA via matching circuits 60 and 70. The output-side external connection terminal OUT is connected to an antenna via a duplexer or the like.
In general, the output impedance of the power amplifier PA is low, e.g., approximately 5Ω, whereas the input impedance of the isolator 100 is high, e.g., approximately 50Ω. The input impedance of the isolator 100 can be lowered by decreasing an angle at which the first and second central electrodes 135 and 136 cross each other and by including the capacitor CS11 as described in Japanese Unexamined Patent Application Publication No. 2007-208943. However, because of a desire to make the isolator 100 smaller, there is a limit as to how small the crossing angle (the input impedance) can be.
Accordingly, the matching circuit 60 including a capacitor C14 and an inductor L13 and the matching circuit 70 including a capacitor C15 and an inductor L14 are provided between the isolator 100 and the power amplifier PA so that the impedance is gradually increased to match the impedance of the isolator 100. However, providing the matching circuits 60 and 70 leads to an increase in insertion loss and an increase in the number of components or cost of a transmission circuit. As illustrated in FIG. 14, the insertion loss reaches 1.2 dB in total as a result of the insertion loss of the matching circuits 60 and 70, which is 0.7 dB, being added to the insertion loss of the isolator 100, which is 0.5 dB.
Also, as a non-reciprocal circuit element of this type, Japanese Unexamined Patent Application Publication No. 2008-85981 describes a non-reciprocal circuit element in which a first variable matching mechanism is connected in series with a plurality of matching capacitors so as to change reactance of the first variable matching mechanism in order to obtain sufficient isolation characteristics in a given frequency band.
However, this non-reciprocal circuit element has a drawback in that insertion loss becomes larger because a high-frequency current passes through the first variable matching mechanism when the high-frequency current is input from the forward direction.